(1) Field of the Invention
The invention relates to digital imaging. More specifically, the invention relates to discrete and continuous time processing of photodata within a single image sensing array.
(2) Background
Image sensing arrays have existed for many years and are generally well known in the art. Generally, a plurality of light sensitive elements such as photodiodes are provided on a focal plane. During a reset phase, a node at one side of the photodiode is driven to a known voltage, while the other side of the diode is connected to a power rail. For example, the diode may be connected between ground and a node driven to VCC during reset. Incident light striking the photodiode causes a photocurrent to be generated which bleeds off that voltage. The photocurrent is continuously varying with time, dependent on the number of photons of the incident light striking the photosensitive element. During a sampling period, a capacitor sources the flow current thereby reducing the charge on the capacitor. The charge on the capacitor at the end of the sampling period is therefore an integration of the photocurrent during the sampling period. This value can then be read out on a bitline. The above is an example of typical discrete time processing used in numerous commercially available image sensing arrays to generate high precision digital images.
A second school of image processing has arisen in which the photocurrent rather than being integrated over a sampling period is processed in continuous time. Some examples are described in Kramer, Sarpeshkar, and Koch, xe2x80x9cAn Analog VLSI Velocity Sensor,xe2x80x9d Proceedings of the IEEE International Symposium on Circuits and Systems, pp. 413-416, 1995, and xe2x80x9cA 590,000 Transistor 48,000 Pixel Contrast Sensitive, Edge Enhancing CMOS Sensor Silicon Retina,xe2x80x9d Andreou and Boahen, pp. 225-239: Proceedings of the 16th Conference on Advanced Research in VLSI, eds. W. J. Dally, J. W. Poulton, and A. T. Ishii, IEEE Computer Society Press, 1995. Continuous time processing of the photocurrent generally yields poor precision and therefore has not gained wide commercial acceptance.
A method and apparatus for continuous and discrete time processing is disclosed. A light sensitive element generates a photocurrent related to an incident light level. The light sensitive element is coupled to both a continuous time processing circuit to process the photocurrent during a continuous segment of time and an integration circuit to generate a digital image related to an average photocurrent during a sampling period.